1. Field of the Invention
This invention relates to heavy duty pneumatic radial tires, and more particularly to a pneumatic radial tire for airplanes having a considerably improved service life by improving the durability of carcass ply.
2. Related Art Statement
Lately, it is demanded to reduce the weight of airplane body in accordance with the demand of energy-saving, and hence it is also strongly demanded to reduce the weight of airplane tire.
Heretofore, it was particularly effective to change a bias structure carcass of the tire into a radial structure carcass as a means for realizing the weight reduction, whereby the number of carcass plies can be decreased to reduce the weight. Furthermore, the use of high strength cord material was effective to reduce the weight of the tire. As a material for such a cord, there is typically an aromatic polyamide fiber.
Since the airplane tires are used under a high loading condition, they are largely deformed to cause a considerable heat build-up. Therefore, they are strongly required to have fatigue resistance of cord strength in the casing and adhesion durability between cord and rubber.
Especially, since the innermost ply constituting the carcass composed of plural plies is subjected to a compressive strain, when the aromatic polyamide fiber having a high modulus of elasticity is applied to a cord for the innermost ply, the resulting cord becomes fatigued due to the compressive strain and the rupture strength decreases, so that satisfactory service durability as the airplane tire can not be maintained. Moreover, the aromatic polyamide fiber is originally low in the adhesion to rubber as compared with the case of nylon or polyester fiber, so that the adhesion force at the shoulder portion is considerably decreased due to the heat build-up, whereby it is apt to cause a separation failure between plies in the belt, while in the bead portion near rim line, the input is very large and the heat build-up is large, so that the adhesion breakage at the boundary between cord and rubber in such a portion is liable to be caused, and consequently sufficient performances as the carcass can not be obtained from a viewpoint of adhesion durability.
As mentioned above, the reduction of weight in the airplane tire by the method of using the conventional high strength aromatic polyamide fiber has drawbacks such as fatigue of strength and degradation of adhesion force, so that the weight reduction of heavy duty pneumatic radial tire can not be realized with the holding of sufficient durability by the above method.